The present invention relates to a semiconductor device and a method for forming those semiconductor devices, more particularly to a semiconductor device and a method for forming the semiconductor device using a conductor material having a low electric resistance and a high resistance to electromigration.
In the case of such a highly integrated semiconductor as a ULSI (Ultra Large Scale Integrated-circuit), the device operation speed, as well as the resistance to electromigration for wiring portions must be increased enough to obtain a high reliability in the long-term use.
As well known, the device operation speed can be increased by reducing the RC delay time of the wiring portions. In order to achieve the above object, therefore, it is required to reduce the electrostatic capacity (C) of the dielectric by using a film with low dielectric constant, as well as to reduce the electric resistance (R) of the wiring material. As the most common wiring material of such an LSI, an aluminum (Al) alloy is used because the alloy is easy to process. The Al alloy is used as Al-0.5%Cu (this means that 0.5% of copper is added in Al. And so forth.), Al-1%Si, etc. The specific resistance of the alloy is about 2.5 to 3.2 .mu..OMEGA.cm.
On the other hand, as the wiring of such an LSI is thinned more and more owing to the improvement of the integration technology, the current density is also increased. As a result, the resistance to the elctromigration of the Al alloy (hereinafter referred to as "resistance to EM") becomes as a subject. The electromigration is a phenomenon that metallic ions of a wiring material are moved by colliding with the flow of electrons to grow voids in the wiring material, resulting in breaking of the wiring. Such a phenomenon is due to grain boundary diffusion in which the metallic ions of the wiring material are diffused mainly at crystal grain boundaries of the metal (hereinafter referred to simply as "grain boundaries"). Thus, the resistance to EM is dominated by the degree of the grain boundary diffusion.
Incidentally, Cu and Ag are used as wiring materials whose specific resistance is smaller than that of the Al alloy. The specific resistance of Cu is 1.8 .mu..OMEGA.cm, so that it has an advantage to increase speed of the device operation. Compared with this, Ag is a metallic material having the lowest specific resistance, but it has a high reactivity to an oxide film and is considered to be difficult to be used as a wiring material of LSI. On the contrary, the reliability of the resistance to EM of the Cu wiring is 2 to 3 times higher than that of the Al alloy wiring. From these points of view, it is now being examined to use Cu as a ULSI wiring material.
As explained above, since the resistance to EM depends on the degree of grain boundary diffusion, a method is well-known to improve the resistance to EM in which impurities or a compounds of the wiring material are precipitated at grain boundaries to block the paths of mass transport and suppress generation of voids in the wiring material. This method is disclosed in Unexamined Published Japanese Patent Application No. 4-364739 (Application No. 3-139871). Although the resistance to EM of Cu is higher than that of Al, generation of electromigration must be further reduced to improve the long-term reliability of the device still more.
When impurities are added in Cu to suppress the generation of electromigration, however, the electric resistance is increased by the impurity scattering due to the added impurities while the resistance to EM is improved. This causes another problem that mere addition of impurities in Cu cannot keep the advantage of using the low resistance Cu in realizing a high speed operation of the device.
Under such the circumstances, it is an object of the present invention to provide a semiconductor device using a conductor material having a low electric resistance and a high resistance to EM and a method for forming such a semiconductor device.